The present invention relates to improvements in X-ray diffractometry. The known methods and apparatuses for struture analysis of crystalline and liquid substances by means of X-rays are based on the considerations and insights of Laue and Bragg; a qualified survey is offered in the book by H. P. Klug and L. E. Alexander, "X-Ray Diffraction Procedures", New York, 2nd edition, 1974.
For investigating single crystals, an X-ray goniometer as described in DE-PS No. 20 41 031 is advantageous in that it is possible to obtain from the adjusted crystal diffraction patterns according to both the de Jong - Bouman method and the Buerger precession method, which was a remarkable advance over the prior art which required two different apparatuses.
In order to study polycrystalline substances, it is customary to employ diffractometers based on a fundamental method of Debye-Scherrer. Using monochromators, detailed structural analyses become feasible with modern automatic diffractometers working on the Bragg - Brentano principle. In special cases, the modified methods of Seemann - Bohlin, Guinier and others are applied.
The schematic view of FIG. 1 shows the Bragg - Brentano modification. A divergent X-ray beam 12 issues either from an X-ray tube ine focus 10 or, less commonly, from the focal line 16 of a (primary) monochromator 14. A specimen 22, usually in the shape of a slab of compressed powder material, is arranged at the center of the so-called goniometer circle 24 so as to be rotatable around an axis that is parallel to the focal line 16 and perpendicular to the circle plane. While the specimen 22 is rotated at a constant angle speed .theta., a unit comprising a slit 26 and a detector 28 is swivelled at exactly double speed (2.multidot..theta.). As the specimen slab 22 is always at a symmetric position relative to the incident and reflected beams, it produces a para-focussing effect which, however, will weaken with increasing angles of .theta. in a wide primary beam. This is due to the fact that the focussing circles, which intersect the focal lines as well as the center of the goniometer circle 24, shrink in size so that conformity of focussing arc and tangent plane (=specimen 22) becomes coarser and finally ceases altogether. Owing to the divergent X-ray beam, the diffractometer circles can be zeroed in only by complex adjustments involving tedious effort. Another drawback is that the para-focussing effect necessitates comparatively large slabs 22 wherein preferred orientations of particles are almost inevitable and, moreover, quite difficult to ascertain exactly.
The Seemann - Bohlin principle is elucidated with the aid of FIGS. 2a and 2b wherein a divergent X-ray beam 12 is seen to be focussed by a monochromator 14 onto a specimen 22 that may be curved or flat. The specimen or sample 22 is arranged at the periphery of the goniometer circle 24, either in a reflection set-up (FIG. 2a) or in a transmission set-up (FIG. 2b); altering the measuring mode thus requires the specimen and the reflex focussing circle to be rearranged, and the reflex profiles differ sharply according to the angular range picked up. For this reason and owing to the inconstancy of the spacing between specimen 22 and detector 28--which latter, therefore, must be moved along the focussing circle by rather intricate kinematic means--this method is more appropriate for film cameras than for automatic diffractometers that are actually intended for measuring the reflexes in a non-stop fashion under conditions as nearly identical as ever possible.
In the arrangements shown in FIGS. 2a and 2b, a primary monochromator 14 will normally be indispensable. By contrast, the set-up of FIG. 1 generally lacks, for reasons of intensity, a primary monochromator but preferably includes a secondary one between detector 28 and specimen 22 in order to eliminate the latter's undesirable radiation such as fluorescent, Compton or radioactive rays the entrance of which into the counter tube commonly used as the detector 28 must be prevented.
By DE-AS No. 1 245 164, a diffraction goniometer has been proposed which aims at using both modes of focussing (FIGS. 2a, 2b) in a single instrument. It provides an auxiliary arm that is rotatable around an axis situated on the goniometer circle and that comprises engaging means for the detector. By arranging the detector such that it is both radially displaceable and coupled to the auxiliary arm's motion, the detector will move on the Seemann-Bohlin focussing circle. However, the instrument affords relatively complex mechanical means due to the coupling and uncoupling of rotatory and translatory motions involved; furthermore, there may be adjustment problems.